.tree-menu ul {
    list-style-type: none;
    padding-left: 16px;
}

.tree-menu li {
    margin: 5px 0;
}

.toggle::before {
    content: '►';
    display: inline-block;
    width: 12px;
}

.nested {
    display: none;
}

.active > .nested {
    display: block;
}

.active > .toggle::before {
    content: '▼';
}